This invention relates to a fluid foil section having improved lift characteristics and, more particularly, relates to a novel fluid foil section having a "nose-down" or stable pitching moment for particular utility in marine propellors.
The thrust produced by a propellor blade configuration having a typical convex forward surface and flat rear surface is comprised of approximately 2/3 thrust due to drop in pressure i.e. suction, over the upper convex forward surface of the blade, primarily near the leading edge, and approximately 1/3 thrust due to a rise in pressure on the flat under rear surface or face of the blade. This distribution of thrust is of little consequence with rigid foils made of aluminum, bronze or steel but, when applied to less rigid foils made of plastics which may flex, can lead to serious problems arising from induced increase in the pitch angle. These problems include an undesirable increase in blade loading, induced drag, reduced revolutions per minute (RPM) and potential blade stall.
A further problem usually inherent with conventional blade configurations is that it is necessary to avoid peak negative pressures on the blade's upper surface which, if allowed to drop below 4 pounds per square inch absolute, causes water vapour to form resulting in cavitation and loss of thrust, coupled with blade damage. To avoid this possibility, the curvature of the convex upper surface of the blade often is reduced. As the amount of curvature corresponds to the amount of lift/unit area, more blade area must be utilized to produce the desired propulsion force. More blade area increases the wetted surface of the blades, and hence the drag, which is a function of the relative water velocity cubed.
Initial attempts to reduce the reliance of the majority of propellor thrust being created by negative pressure appear in U.S. Pat. No. 3,635,590 granted Jan. 18, 1972. While this configuration appears to transfer a good proportion of the lift to a positive pressure on the face of the blade, the following undesirable features precluded this design from competitive utility. The upper, or forward surface, in common with the lower surface, were essentially planar with a convex surface near the leading edge. Since such surfaces produce a minimum of lift, their incorporation into blade designs was limited in utility. A nose-up pitching moment remained, permitting unproductive blade flexing of plastic blades to occur.
U.S. Pat. No. 3,697,193 granted Oct. 10, 1972 discloses a propellor blade configuration in which an attempt was made to increase the distribution of pressure to a more positive value on the face of the blade. The modified blade not only increased the pressure loading on the face of the blade, it moved the pressure loading towards the trailing edge. However, the following problems remained. The upper and lower surfaces were still essentially planar. Subsequent analysis and tests of the fluid foil section revealed that there was a positive pressure produced on the face of the blade immediately adjacent to the leading edge, which would preclude a desired nose-down pitching moment of the blade foil.